1. Field of the Invention
The present invention relates to a method for producing lithium hexafluorophosphate, and specifically relates to a method for producing lithium hexafluorophosphate useful as an electrolyte for lithium secondary batteries and a catalyst for organic synthesis.
2. Description of the Related Art
Conventionally, lithium hexafluorophosphate is produced by the following steps:
1) Lithium fluoride (LiF) is dissolved in hydrofluoric acid anhydride to give a solution.
2) PF5 gas is blown into this solution. This operation causes LiF to react with PF5 to produce lithium hexafluorophosphate.
3) The solution is cooled to deposit lithium hexafluorophosphate.
4) The solution is filtered to separate the deposited lithium hexafluorophosphate.
5) The separated lithium hexafluorophosphate is then dried to yield lithium hexafluorophosphate as a product.
6) Following this step, a product of lithium hexafluorophosphate is dissolved in an organic solvent, for example, to for use as an electrolyte for a lithium battery.
However, according to the conventional related method for producing lithium hexafluorophosphate, many kinds of impurities such as lithium fluoride and oxyfluoride acid compound are contained in hexafluorophosphate.
When lithium hexafluorophosphate containing such impurities is dissolved in an organic solvent for use as an electrolyte for a lithium battery, lithium fluoride contaminate is not soluble in the organic solvent, thereby requiring filtering of the electrolyte.
Examples of methods for removing these impurities are a method using an ion exchange resin where lithium hexafluorophosphate is dissolved in organic solvent to remove impurities (Japanese Published Unexamined Patent Application No. 1984-87774) and a method where neutralization treatment is carried out in an organic solvent (1984-81870.) However, these methods are complex in operation and provide low productivity.
As described above, related methods for producing lithium hexafluorophosphate require complex after-treatment. Even if after-treatment is performed, lithium hexafluorophosphate of high purity cannot be produced, which is problematic.